The present invention relates, in general, to examining body parts and, in particular, to an apparatus and method by which the mechanical stiffness of body tissue being investigated is determined from ultrasonic transmissions to and reflections from the body tissue.
At the present time, ultrasonic imaging is the second largest medical imaging modality after X-ray imaging. In ultrasonic imaging, images are formed by transmitting high frequency acoustic waves into the body and then appropriately mapping the response of returning echoes as the original acoustic signal propagates into the body. An acoustic echo is generated at each interface within the body, which is characterized by an impedance discontinuity. Typically, an image is obtained by mapping the intensity of the returning echo signals as a function of range and direction of propagation. Movement of the ultrasonic waves within a plane allows one to analyze sequentially tissue responses from a large number of directions. Images developed in this manner are known as B-mode images (i.e., xe2x80x9cbodyxe2x80x9d mode images).
Other ultrasonic imaging techniques are in practice at the present time. Color Doppler mode is one such other technique. Color Doppler mode is a methodology in which the mean Doppler frequency shift imposed upon the returning acoustic echoes by moving target structures, such as the blood, is measured and mapped. The mean Doppler shift is determined by measuring the mean phase rotation or time delay between successive acoustic pulses in a series of pulses known as a packet. Likewise, Power Doppler is a mode in which the intensity of the Doppler signal, rather than the mean frequency shift, is mapped to form an image.
Recently, an imaging mode, known as harmonic imaging, has been introduced. In this method, an ultrasonic pulse is transmitted into the body as with conventional B-mode imaging. Instead of sensing the return of acoustic echoes at the same frequency as the original pulses, filtering techniques are used to sense signals at harmonic frequencies. The intensity of these sensed signals is then mapped in a conventional manner. Because these signals are generated as a function of the non-linear propagation characteristics of the tissue, different anatomical features can be observed; perhaps with better contrast
In a recently published paper entitled xe2x80x9cInvestigation of Real-time Remote Palpation Imagingxe2x80x9d by Nightingale, Soo, Nightingale, Palmeri and Trahey, Proceedings SPIE Medical Imaging 2001, there is described an experiment in which tissue was first insonified with a conventional ultrasonic pulse and the radio frequency signal associated with the returning acoustic echo then was recorded. Next, the tissue was insonified with a continuous (i.e., relatively long) acoustic wave (120-300 W/cm2) that generated a force within the tissue. Then, the displacement of the tissue resulting from this force was measured using a radio frequency cross-correlation technique between the initial ultrasonic pulse and a second ultrasonic pulse. A displacement, from the resultant force, of as much as 30 microns could be observed. Maximum displacements were generally obtained within 5 ms. The tissue displacements correlated well with B-mode image anatomical structures. The amount of displacement and the recovery time can be associated with the stiffness properties of the propagation media.
This displacement phenomenon can be explained in terms of the physics of wave propagation. When a wave travels in a medium, be it an acoustic wave or an electromagnetic wave, it carries with it not only energy (E) but also momentum (P). As the acoustic wave propagates into tissue, however, energy is absorbed due to inelastic transport processes. Associated with this energy loss is a commensurate change in momentum. Momentum changes also can occur when energy is reflected from acoustic interfaces. This may be an elastic process.
From Newton""s Laws, this momentum change imposes a force on the differential tissue volume in the path of propagation (dP/dt=F). This force, in turn, causes the infinitesimal tissue volume to move, F=massxc3x97acceleration. The extent of the movement is a function of the stiffness of the material as well as the local absorption.
In its simplest form, the present invention may employ algorithms and hardware similar to those that have been used previously in Doppler imaging to display images of the movement of body parts. In the present invention, the ultrasonic signals that are transmitted to the target not only are reflected for developing images of the target from the reflections, but, by appropriate selection of the intensity of the transmitted signals, the body tissue being investigated is deformed or moved when the transmitted signals impinge on the target to measure the displacement due to ultrasonic wave propagation. The deformation or movement of the body tissue being investigated is imaged and is representative of the mechanical stiffness of this body tissue.
Apparatus for indicating mechanical stiffness properties of body tissue, constructed in accordance with the present invention, includes transmitter means for transmitting to a target in a body (a) a first ultrasonic pulse having a first acoustic intensity sufficient to deform the target, and (b) subsequently a second ultrasonic pulse having a second acoustic intensity, different from the acoustic the intensity of the first ultrasonic pulse, sufficient to deform the target. This apparatus also includes receiver means for receiving (a) a reflection from the target of the first ultrasonic pulse and developing a first signal representative of the position after deformation of the target caused by the first ultrasonic pulse, and (b) subsequently a reflection from the target of the second ultrasonic pulse and developing a second signal representative of the position after deformation of the target caused by the second ultrasonic pulse. This apparatus further includes indicating means responsive to the first signal and the second signal for indicating the change of deformation of the target caused by the second ultrasonic pulse relative to the deformation of the target caused by the first ultrasonic pulse.
A method for indicating mechanical stiffness properties of body tissue according to the present invention includes the steps of transmitting to a target in a body a first ultrasonic pulse having a first acoustic intensity sufficient to deform the target, receiving a first reflection from the target of the first ultrasonic pulse, transmitting a second ultrasonic pulse having a second acoustic intensity, different from the acoustic the intensity of the first ultrasonic pulse, sufficient to deform the target, and receiving a second reflection from the target of the second ultrasonic pulse. This method also includes the steps of developing from the first reflection a first indication of deformation of the target caused by the first ultrasonic pulse, developing from the second reflection a second indication of deformation of the target caused by the second ultrasonic pulse, and developing from the first deformation indication and the second deformation indication an indication of the deformation of the target caused by the second ultrasonic pulse relative to the deformation of the target caused by the first ultrasonic pulse.
It is to be understood that the foregoing general description of the present invention and the following detailed description of the present invention are exemplary, but are not restrictive of the invention.